Solid propellants with stability enhanced additives of particulate refractory carbides or oxides

ABSTRACT

Ammonium perchlorate propellants utilizing a polybutadiene binder provide a smokeless exhaust and burn stably in a motor at a burning rate above 0.40 in/sec at 1,000 psia with no combustion instability if they include 0.25-5% of refractory metal carbides or oxides and carbon in the form of hollow, broken or unbroken carbon spheres, carbon particles or carbon flakes.

Unite States atent [19] Cohen et al.

[ Dec. 9, 1975 SOLID PROPELLANTS WITH STABILITY ENHANCED ADDITIVES OFPARTICULATE REFRACTORY CARBIDES OR OXIDES [75] Inventors: Joseph Cohen;Gilbert A.

Zimmerman, both of Sacramento,

Calif.

[73] Assignee: Aerojet-General Corporation, El

Monte, Calif.

[22] Filed: June 7, 1973 [21] Appl. No.: 360,867

[52] US. Cl. 60/219; 149/2; 149/19.1; 149/20; 149/21; 149/76; 149/199;149/110 [51] Int. Cl. C06D 5/06 [58] Field of Search l49/19.9, 20, 44,76, 21, 149/2, 19.1; 60/219 [56] References Cited UNITED STATES PATENTS3/1960 Fox 149/87 X 3,666,575 5/1972 Fisher 149/19.2 3,734,786 5/1973Walden et al.......... l49/l9.9 X 3,822,154 7/1974 Lawrence et al.149/20 X Primary ExaminerBenjamin R. Padgett Assistant Examiner-E. A.Miller Attorney, Agent, or FirmEdward O. Ansell; Marvin E. Jacobs [57]ABSTRACT Ammonium perchlorate propellants utilizing a polybutadienebinder provide a smokeless exhaust and burn stably in a motor at aburning rate above 0.40 in/sec at 1,000 psia with no combustioninstability if they include 0.255% of refractory metal carbides oroxides and carbon in the form of hollow, broken or unbroken carbonspheres, carbon particles or carbon flakes.

14 Claims, 2 Drawing Figures atsnt Dec. 9 1975 3,924,405

i \D a 5 I000 3 J3 \D LL] M a O 5 lo \5 2o T\ME., sEcoNDs o 246810l2l4k6l820 Tnvui, SECONDS;

SOLID PROPELLANTS WITH STABILITY ENHANCED ADDITIVES OF PARTICULATEREFRACTORY CARBIDES OR OXIDES BACKGROUND or THE INVENTION 1. Field ofthe Invention vThe present invention relates to stable burning,smokeless propellantsand more particularly to high en ergy, ammoniumperchlorate propellants based on a polybutadienebinder.

'2. Description of the Prior A c a The absence of a visible exhaust froma solid rocket motor is a highly desirable attribute, particularly formilitary applications. Such' performance is possible by eliminating fromthe propellant formulationany material which will form a solidparticulate on combustion (primary smoke). Double-base(nitrocellulose-nitroglycerin) compositions have been the principalpropellants used for smokeless applications. Although more desirablebecause of higher performance,.the composite propellants based onammonium perchlorate in an organic binder have used materialswhic'hjform solid particulates, principally aluminum, to eliminate'combustion instability and maximize specific impulse. Eliminatingaluminum from the composite system eliminates primary smoke but bringsin the problem of combustion instability when the'propellantsareforrnulated with high oxidizer content for high specific impulse.

Recent work has shown that smokeless ammonium perchlorate (AP)propellants using 'a hydroxy-terminated polybutadiene (HTPB) binder willyield a smokeless exhaust (primary smoke) and burn stably in a motor ifthe burning'rate is about 0.40 in/sec or lower at 1,000 psia. At burningrates above this level, com-' Smoke is defined in terms ofsolid-propellant exhaust as including all visible signature effects withthe exception of flash or luminosity effects. Smoke is more strictlyconsidered to be of two general categories: either primary, whereinsolid particles-in the propellant exhaust affect its lighttransmissivity independently of the environment, or secondary (induced),wherein some of the gaseous components in'the exhaust such as HCl, HF,NO, or condensible water vapor interact with the ambientair to producevisible aerosols of liquid or solid particles. Sources of primary smokefrom the propellant include unburned carbon and metal oxides.

The selection of anyprop'ellant involves the determination ofperformance factors, safety factors, life factors and cost factors.Performance factors to be considered include specific impulse, densityand thermal expansion characteristics, mechanical properties, burningrate, combustion stability, sensitivity of chamber pressure to graintemperature and propellant erosivity. Safety factors include sensitivityto impact,"friction, dropping, fire and spark. Also to be consideredunder safety are thermal stability or auto ignition temperature,processing hazards, toxicity and exhaust product toxicity. Life factorsinclude polymer degradation, moisture sensitivity, plasticizer migrationand catastrophic phenomena connected with grain cracking and bondfailure. Previously, smokelessness resulted in definite penaltiesin oneor more of these factors or determinants of the factors. f

Combustion instability is a complex phenomena involving the combinationof the inner motor configura- 5 one or more frequencies, the acousticenergy added to the system by the propellant exceeds that which isdissipated by frictional damping or carried from the chamberconvectively. Because the phenomena does involve the interrelation ofmotor configuration and propellant properties and because theseinteractions are not completely understood, it is not always possible tospecify propellant'or chamber design procedures which will guaranteestable bumingl Presently the primary problem in the use of smokelesspropellants is combustion instability. For many years the use ofhighipercentages of aluminum in solid propellants almost completelyinhibited combustion instability. The removal of aluminum to make .theprimary exhaust smokeless causes the propellant to exhibit unacceptabletendencies toward unstable com- .bustion.

OBJECTS AND SUMMARY 'OF INVENTION It istherefore an object of theinvention to. provide a smokeless propellant in which combustioninstability is substantially suppressed.

A further object. of the invention is to provide a propellantsubstantially free of primary smoke in the exhaust and exhibiting a highspecific impulse and burning rate without exhibiting any combustioninstability.

Yet another object is the provision of an ammonium perchlorate loadedpropellant that is absent aluminum and whichmaintains combustionstability at a burning rate greater than 0.40 in/sec at a pressure ofabout 1,000 psia.

lnaccordance with the invention it hasbeen discovered that the additionof small amounts of additives selected from refractory metal carbides oroxideswill provide a stableburning smokeless propellant for somechamber-propellant interactive resonant frequencies and at a burningrate above 0.40 in/sec. When a small amount of carbon in the form ofhollow, thin walled spheres, whole or broken, or flakes, is also addedthe regime of resonance frequencies for stable combustion followingdetaileddescription when'considered in conjunction with the accompanyingdrawings.

BRIEF DESCRlPTlON OF THE DRAWINGS FIG. 1 is a graph showing the firingcurve .for a'dual thrust motor in which theboosterpropellant grain ofthe bipropellant configuration contains additives according to theinvention; and

FIG. 2 is a graph showing the firing curve for a propellant grainwithout additives in the booster grain.

DESCRIPTION OF- THE PREFERRED EMBODIMENTS The propellant compositionusually contains a high proportion of combustible solids, typically inexcess of by weight, a small proportion of binder, usually below 15% byweight, and a small amount below 3% by nate,

weight of burning rate accelerator. The combustible solids usuallycomprise an oxidizer such as ammonium perchlorate, HMX or RDX and 0.2%by weight of the combustion stabilizing solid added in accordance withthe invention.

Preferred binders are elastomeric hydrocarbon polymers formed by thechain extension and cross-linking reactions of functionally terminatedliquid polybutadiene polymers. Such polymers may includecarboxy-terminated polybutadiene cured with amines or epoxides,polybutadiene acrylonitrile-acrylic terpolymers cured with epoxides andhydroxy-terminated polybutadiene cured with diisocyanates.Hydroxy-terminated polybutadienes are preferred due to cost, reactivity,availability'considerations and mechanical properties. The butadiene maybe derived from the lithium initiated polymerization (Li-l-lTPB) or freeradical initiated polymerization (FR-HTPB).

The composition may also contain a minor amount below of variousadditives such as cure promoters, stabilizers and thixotropic controlagents, or reactive polymeric modifiers such as one or more diols orpolyols. The isocyanate is generally presentin at least an equivalentamount sufficient to react with the hydroxy prepolymer and hydroxylsubstituted modifiers.

The equivalent weight of the liquid prepolymer is at least 1,000 and notusually more than 5,000. The functionality of the polymer isadvantageously from about L7 to about 3.0, preferably from about 1.9 to2.3 to form by cross-linking and chain extending elastomeric polymers ofmolecular weight of at least 30,000. Since higher molecular weightprepolymers may require heat to reduce viscosity, the molecular weightis preferably from 1,000 to 4,000.

The polyisocyanate for curing the prepolymer can be selected from thoseof the general formula (R(NCO),,, in which R is a dior polyvalentorganic radical containing from 2-30 carbon atoms and m is 2, 3 or 4. Rcan be alkylene, arylene, aralkylene or cycloalkylene. It is preferredthat the organic radical be essentially hydrocarbon in characteralthough the presence of unreactive groups containing elements otherthan carbon and hydrogen is permissible as is the presence of reactivegroups which are not capable of reacting with isocyanate groups capableof forming urea or carbamate linkages such as to interfere with thedesired reaction.

Examples of suitable compounds of this type include benzene-l,3-diisocyanate, hexane-l ,6-diisocyanate, toluene-2,4-diisocyanate(TDl), toluene-2,3-diisocyanate, diphenyl-methane-4,4'-diisocyanate,naphthylene-l ,S-diisocyanate, diphenyl-3 ,3 '-dimethyi-4,4diisocyanate, diphenyl-3 ,3 '-dimethoxy-4,4'-diisocyabutane-1,4-diisocyanate, cyclohex-4-ene-1 ,2- diisocyanate,benzene-1,3,4-triisocyanate, naphthylene-l ,3 ,5,7-tetraisocyanate,metaphenylene diisocyanate (MDl), isocyanate terminated prepolymers,polyaryl polyisocyanates and the like.

Polyols are preferably, but not limited to, diols or triols and can beeither saturated or unsaturated aliphatic, aromatic or certain polyesteror polyether products. Exemplary compounds include glycerol, ethyleneglycol, propylene glycol, neopentylglycol, pentaerythritol,trimethylolethane, glycerol triricineolate, or alkylene oxide adducts ofaniline such as lsonol which is N,N- bis-(Z-hydroxypropyl) aniline andmany other polyols well known in the art which can be incorporated intothe binder composition to control the degree of crosslinking. Theparticular compound and amount utilized is dependent on thefunctionality and nature of the hydroxyl terminated prepolymer andpolyisocyanateemployed in the binder composition.

Since the functionality of .Li-HTPB is generally slightly less than 2,the polyol is preferably a trio] so as to provide cross-linking betweenpolymeric chains upon reaction with isocyanates. As exemplary polyols,mention may be made of glycerol triricinoleate (GTRO) and lsonol (apropyleneoxide adduct of aniline), N,N-bis-(Z-hydroxypropyl)-aniline.The polyisocyanate is present in an amount necessary to satisfystoichiometry, that is, the functionality of the HTPB and any otherpolyol present in the composition. The polyisocyanate may be a di-,trior higher functional material and may be aliphatic in nature such ashexanediisocyanate but is preferably an aromatic polyisocyanate such asTDl. A catalytic cure promoting agent can be utilized. These agents maybe metal salts such as metal acetylacetonates, preferably thoriumacetylacetonate (ThAA) or iron acetylacetonate (FeAA).

The combustion stability promoting additives in acc'ordance with theinvention may be used alone but are preferably used in combination atconcentrations as low as 0.2% as single ingredients or combined. Whileno upper limit is theoretically non-functional, however, with respect todegradation of performance and optimum exhaust smoke characteristics,the concentration of the solid additives should not exceed about 3% byweight of the propellant composition. The refractory metal carbide oroxide should have a melting point of at least about 2,000C.

Suitable high melting materials are the carbides and oxides of metalsincluding thorium, tungsten, silicon, molybdenum, aluminum, hafnium,vanadium. The refractory compound should be provided in the form of fineparticles ranging between 2 to 10 microns. The use of carbon inrefractory metal compounds such as zirconium carbide will have a minimalaffect onv smokeless performance. Carbon will, of course, burncompletely to CO and CO, while zirconium carbide at a level of 0.5% willproduce about 0.7g of solid ZrO, per g of propellant burned. Smokemeasurements made on firings of a propellant formulated with additiveand one formulated without showed that the light transmission throughthe exhaust plume to be the same for both propellants, demonstratingthat the ZrC had no measurable effect on the amount of primarysmokeproduced.

it is believed that both the carbon and ZrC function through aparticulate damping mechanism. Further, the carbon and ZrC represent twodifferent classes of material. One functions as a particulate damperclose to the burning surface. Carbon is completely consumed in thecombustion process and cannot act to provide particulate damping theentire time the gas is present in the motor. The other, ZrC, is aparticulate which is present in the gas phase either as ZrC or ZrO mostprobably as ZrC. I

The carbon additive when utilized in combination with the refractoryoxide or carbide can have diverse physical form and size. However, whenthe carbon is utilized alone as a combustion stabilizer, it shouldpreferably have a thickness between i and about 10 mierons and a lengthof between about 25 to 400 microns. A preferred form of carbon is smallparticles such as platelets or spheres or carbon powder such as Thumax(0.3 ,W hen in the form of flakes or platelets, the preferred sizes are10 to a X i to 8 p. thick.

Table 1 Property A-100 A-200 App. mean diameter, microns 110 200Diameter range, microns 75-150 150-250 Wall thickness, microns 2-3 3-8Bulk Density, g/cc 0.10-0.25 0.07-0.20 Particle density, g/cc 0.l50.400.15-0.35

Theoretically, the burning rate of a propellant is dependent only on thechamber pressure. Actually it is also dependent on the velocity of gasflow over the burning surface. The higher the gas velocity across thepoint on a grain, the higher the burning rate at that 6 pears thaterosive and unstable burning are related phenomena.

Combustion instability of the candidate smokeless propellants inaccordance with the invention was studied in a T burner which is astandard device for experimental measuring of combustion instability.The T burner device uses opposing cylindrical grains and is usuallyoperated at pressures of 500 and 1,000 psi. The chamber length wasvaried to provide fundamental acoustic frequencies near 3,000 and 4,000Hz. The tests were utilized to determine the following parameters:

01,, growth constant for acoustic pressure AP amplitude of acousticpressure oscillations R, response function, ratio of burning rate changeto pressure change Cylindrical grains were formulated with 12 parts ofan hydroxy-terminated polybutadiene binder system containing astoichiometric amount of TDI and an appropriate amount of ammoniumperchlorate and different additives. The composition was formed intocylindrical grains suitable for the T burner test and the results of thetestare provided in the following table.

Table 2 Frequencies 2000 Hz 3000 Hz 4000 Hz Example AP Additive rin./sec.

No. Wt%Type Wt% at 1000 psia a, AP R,, a, AP R, a, AP R 1 88 None 0 .42115 1.33 12 18 .15 Stable 2 88 None 0 .58 70 75 1.73 63 105 1.02 Stable.16 3 88 Aluminum 0.5 .55 47 60 .55 Stable 13 4 88 A1 0 1.0 .64 60 601.19 34 60 .66 30 7 .39 5 88 Broken Carbon 1.0 .64 52 65 1.06 1 .145Stable .14

Spheres 6 87 P-33 Carbon 1.0 .51 54 160 1.52 28.5 50 .7 Stable 2 7 87(Broken Carbon 0.5) .49 36 60 1.15 .21 Stable .18

(Spheres (ZrC 0.5) 8 88 ZrC .5 .57 53 70 1.35 =0 .24 Stable .14

point. Some propellants are more susceptible to erosive burning thanothers. In general, erosive burning is more prevalent in lower burningrate propellants than in those with high burning rates.

Unstable burning is a phenomena common to all propellant systems yet notto all propellants within a system. Furthermore, additives which in onesystem may control combustion instability may have no affect or anadverse affect in another propellant or binder system. It appears thatunstable burning is more common with higher energy propellants than withlower energy propellants. Tests have indicated that unstable burning isa result of the production of transverse or longitudinal acousticaloscillations of the combustion gases during burning. These oscillationsresult in areas of high and low velocity around or along the grain whichhave a marked effect on the local burning rate. At a high velocity areacaused by oscillation of the gas, the burning rate rises rapidly,causing a further increase in pressure. At a low velocity or nodalpoint, the burning rate is very low. It may be seen that the non-uniformburning of the grain can cause premature break-up even if the averagechamber pressure does not exceed the maximum chamber design pressure.Extremely uncontrolled performance and chamber failures are commonlyassociated with aggravated, uncontrolled resonance or unstable burning,although in some rockets it can be detected only by high frequencyinstrumentation. 1t ap- The higher burning rate propellant withoutadditives, Example No. 2, is more unstable at 3,000 Hz, i.e., higher a,AP and R The propellants containing broken carbon spheres (Example 5) orzirconium carbide (Example 8) or these additives in combination (Example7) eliminate the instability at 3,000 Hz and above with some-benefitobtained at 2,000 Hz, particularly in the reduced response function(R,,). Formulation No. 6 including a standard amorphous, rubber grade ofcarbon black, P-33, shows some reduction of the instability at 3,000l-izv but is not as effective as the carbon in the form of broken,hollow spheres (Example 5).

This decrease in combustion instability shown in T burners has beenverified in motor firings of a dual thrust configuration where a boostergrain composed of 88% ammonium perchlorate (AP) in an HTPB binder with0.5% of the zirconium carbide (ZrC) was used. Although some instabilitywas observed as shown by the DC shift, this shift was only 10% of thatshown by the propellant without additive. Further, the onset of theshift was delayed until the end of the boost phase.

A second motor was fired using the combination of 0.5% ZrC and 0.5%partially broken carbon spheres, formulation No. 8 above, in the boosterpropellant. The results were even better with the second motor. The DCshift was eliminated entirely, leaving a residual pressure coupledmaximum amplitude of only 10 psi at the operating pressure of 1,200 psi.This minor instability is well within acceptable operation limits forsolid rocket motors. The firing curve for this dual thrust motor isshown in FIG. 1, where formulation No. 8 was used for the boost phase ofthe operation. FIG. 2 typifies the performance of the compositionwithout additives showing the large pressure spikes resulting fromcombustion instability. Further T burner date were obtained on theeffect 1% ZrC (no carbon) on stability of the same propellant used forevaluating the 0.5% mixture with carbon and also on the effect of usinga lower percentage of ammonium perchlorate as a lower burning ratepropellant. The results are shown in the following table.

8 2000 and 3000 Hz although they stabilized at a frequency of 4000 Hz.It is evident that at higher burning rate, i.e. batch No. 18,instability is increased at 3000 Hz.

The effect of various forms of carbon is seen in batches 14, 29, 13 and15. Neither P-33 (Example 14) norcarbon fibers (Example gave animprovement in stability at 87% AP. Carbon spheres A-l00 (Example 13)gave improved stability at 87% AP and 3000 Hz. At 88% AP, carbon spheresgave improved stability at both 3000 or 4000 Hz.

Zirconium carbide (Example No. 16) gave signifi- The data given in theabove table shows that 1% ZrC (Example No. 9) is equivalent inperformance to the propellant containing the mixture of additives(Example No. 7). The use of some carbon is considered significantlysuperior since it does not create any particulate smoke. The firings ofthe propellants of Examples No. 10 and l l were both stable until theratio of S /S was 1 or less and then the firing became unstable. Sindicates the area of the propellant burning and S is thecross-sectional area of the chamber. The lower burning rate propellantcontaining a lower amount of ammonium perchlorate as shown in ExampleNo. 12 shows that with this composition stability is improved, beingstable at 2200 and 2600 Hz. Further T burner data is shown in thefollowing table.

cantly improved stability at 3000 and 4000 Hz. Additional testing ofcarbon spheres, 1%, and ZrC, 0.5%, as single additives in the T burnerand also in motors (Examples 19, 20 and 21) showed these compositions tobe unstable in the T burner at 2500 Hz. Motor firings of Example 21 alsoshowed these compositions were unstable when the propellant web burnedout to a diameter corresponding to a frequency of 4000 at 5000 Hz.

In combination the ZrC and carbon spheres (Examples 25, 19, 26, 23 and24) gave stable combustion in the T burner at 2500 Hz and were alsostable in 9-inch O.D. grains, when fired in motors having a frequency atburnout of 2700 Hz. The effect of the combination produces animprovement in stability over that shown by the single ingredients whenused alone.

TABLE 4 a 500 psia 900 psia Ex. AP 1000 2000 Hz 3000 Hz 4000 Hz 2500 HzNo Wt% Additive Wt% psia a, AP R, a, AP R,, a, AP R, a, AP R,,

13 87 C Spheres l 0.56 32 7 .l3 38 5-10 .475 14 87 P-33 1 0.51 54 1501.52 28.5 .7 Stable 2 15 87 Carbon Fibers 1 0.49 47 130 1.38 41 .8Stable .18 16 88 ZrC 0.5 0.56 53 1.35 O .24 Stable .14 17 88 0.42 40 1151.33 12 18 .15 Stable 18 88 0.58 70 1.73 63 1.02 Stable .16 19 87 CSpheres 1 0.53 200 450 1 20 88 C Spheres l 0.41 200 450 l 21 88 ZrC 0.50.56 200 500 1.1 22 88 C Spheres/ZrC 0.5/0.5 0.41 Stable 23 88 CSpheres/ZrC 0.5/0.5 0.41 Stable 24 87 C Spheres/ZrC 0.5/0.5 0.58 Stable25 87 C Spheres/ZrC 0.5/0.5 0.49 Stable 26 87 C Spheres/ZrC 0.5/0.5 0.54Stable 27 87 Thcrmay/ZrC 0.5/0.5 0.61 Stable 28 87 P-33/ZrC 0.5/0.5 0.59Stable 29 87 C Spheres 1 0.67 52 65 1.06 4 .145 Stable .13 30 87 CSpheres/ZrC 0.5/0.5 0.54 Stable 31 87 C Spheres/ZrC 0.5/0.5 0.55 Stable32 87 Thermay/ZrC 0.5/0.5 0.61 600 33 87 P-33/Zrc 0.5/0.5 0.59 153 60034 87 C Spheres/ZrC 0.5/0.5 0.54 171 500 35 87 C Spheres/ZrC 0.5/0.50.55 163 500 100% broken spheres 100% unbroken spheres 2600 Hz Thecontrol propellant batches l7 and 18 show that without additives all APpropellants are unstable at A further evaluation of the effect of carbonand ZrC was tested in Example Nos. 27, 28 and 29 which in motor firingsevidently because at an S /S ratio of l the grains are burning as flatslabs and have no contribution from side wall burning as is the case ina typical lD burning grain configuration.- The results show thatstability above 2500 Hz is primarily due to the combi- Example N0. 36(89% AP, r 0.41 in/sec at 1000 psia) containing no additive was unstableboth in the T burner at 2500 Hz and in the motor at even a higherfrequency of -4000 Hz.

Example No". 37 (88% AP, 0.5% Fe O r= 0.59 in/- sec at 1000 psia) againwas unstable at a higher frequency'in the motor indicating the effect ofhigher burning rateon the instability.

Example No. 38 (87% SP, r 0.49 in/sec at 1000 psia) contained 100p,carbon spheres and 5p. ZrC and was: found tobe stable both in the Tburner and in the motor'firing down to 4800 HZ, illustrating the effectof the combination of additives. I I Example No. 39 (87% AP, r 0.54in/sec at 1000 nation, carbon plus'ZrC, and is not'dependent on thepsia) contained 200p. carbon spheres and 5p. ZrC and form of carbon. wasfound to be stable in the T burner illustrating that The experimentssummarized in Table 5 show the 200p. carbon spheres are as effective asthe 100p.

comparison of T burner results at 900 and 2500 psia spheres.

with full scale motor tests conducted at 900-1500 psia Example No. 40(88% AP, r 0.56 in/sec at 1000 and 70F. The binder in each example was aplasticized psia) contained only 0.5% ZrC and was found to be un- HTPB.stablein the T'burner and at -5000 Hz in the motor il- Table 5 FullScale vs T Burner Test Data on Smokeless Composite Propellants Burning TBurner Tests Example AP Additive Rate 2500 Hz & 900 psia Full ScaleMotor Tests No. Wt.% Type I Wt.% Size, p. in/lb a, AP R, Type Gr. O.D.Results 36 89 None .41 =200 900 1] 7.75 Unstable at 6" Dia. 37 88 Fe,0.5 1 .59 160 550 1.1 [l] 5.00 Unstable at 4 7 Dia. 38 87 C/ZrC .5/.5200/5 .49 Stable .07 [1 5.00 Stable 39 87 C/ZrC .5I.5" 200/5 .54 Stable.Not Tested 40 88 ZrC .5 5 -56 =200 ===S00 1.l [l] 5.00 Unstable at 4.8"

' Dia. 41 87 C Spheres l 200 .53 =200 450 1 Not Tested 42 88 C-Spheres l200 .41 ==200 =450 1 [2] 9.90 Unstable at 6" Dia. 43 88 C/ZrC .5/.5200/5 .41 Stable [2] 9.00 %table at 5" v ia." 44 88 C/Zrc .5/.5 200/5.41 Stable (2600 [2] 9.00 Stable a 9" H 150 300 (1200 Hz) 45 87 C/ZrC.5/.5 200/5 .67 =23Q 300 I Not Tested 46 87 C/ZrC .5/.5 200/5 .64 ==l87900 Not Tested 47 87 C/ZrC .5/.5 200/5 .58 Stable (2600 [2] 9.00 StableSolid Strand at 1000 psia "Lost Nozzle insert [1] 100 lbs. of propellantGrain Design-A [2] 100 lbs. of propellant Grain Design 8 The correlationbetween motor diameter and motor resonant frequency is shown in thefollowing table:

' Motor Resonant Frequency, cps

Motor Diameter,

. D, inch oooqoco lustrating the need for the carbonspheres forstability.

Example No.41 (87% AP, r.= 0.53 in/sec at 1000 psia) contained 1% of200g. carbon spheres and was found to be unstable in the T burnerillustrating the need for the ZrC in combination.

Example No.42 (88% AP, r 0.41 in/secat 1000 psia) contained 1% of 200p.carbon spheres and was found to be unstable in the T burner and in themotor at -4,000 l-lz illustrating again the need for the combination toachieve stability.

The remaining batches illustrate the effect of the combination of carbonspheres and ZrC. Both the T burnerand motor results show theeffectiveness of the combination of additivesiin achieving stabilityover the range of burning rates from -0.40 to -0.60 in/sec at 1000 psiaat frequencies as low as 2500 Hz and an oxidizer level from 87 to 88%.At the burning rates above 0.60 in/sec at 1,000 psia instability at 2500Hz was evident in the T burner. Stability in motors was maintained overthe temperature range of 40 to +l35F as l. an oxide or carbide having amelting point of at shown by the motor fired from Example No. 47.

Thus it is apparent that solid additives such as a resisting of thorium,tungsten, silicon, molybdenum, fractory metal carbide alone, irregularthin carbon paralurninum, hafnium, zirconium and vanadium oxticles suchas broken carbon spheres or the combina- 5 ides and carbides; and tronof refractory metal compound with diverse forms 2. particulate carbon.of carbon are capable of providing stable burning, high 2. A compositionaccording to claim 1 in which the energy, smokeless propellants withoutsignificant loss binder is an elastomeric hydrocarbon polymer present ofspecific impulse even though aluminum has been in an amount of no morethan by weight and the eliminated from the fuel. additive ispresent inan amount from 0.1 to 4% by A further series of T burner data forpropellants conweight. taining other refractory compounds such as 0.5weight 3. A composition according to claim .2 in which the percent ofhafnium oxide, niobium carbide or tantalum binder is a chain extendedand cured liquid polybutadicarbide in combination with 0.5 weightpercent of 200 ene polymer having an equivalent weight between microndiameter carbon spheres and 87% ammonium 15 1,000 and 5,000 and afunctionality between L7 and perchlorate (AP) is presented in thefollowing table. 3.0.

' Table 7 Motor Resulting Example Burn Rate Pressure, ResonanceResonance,

No. Additive in/sec psig design f, Hz f, Hz a, sec AP, psig 48 mo, .571084 2600 0 Stable 0 49 HfO, .58 1072 2200 2300 +160 765 50 NbC .59 10882600 0 Stable 0 51 NbC .53 1066 2200 0 Stable 0 52 TaC .53 1044 2600 0Stable 0 53 TaC .54 1100 2200 2250 +147 670 Example 54 compositionaccording to claim 3 in which-the OXlCllZlng salt is ammoniumperchlorate present in an A Propellant was compounded as followsl vamount of between 85% and 90% by weight.

5. A composition according to claim 4 in which the additive comprises0.2 to 1% by weight high melting carbide having a particle size rangingbetween 2 to 10 Ingredient Wt.%

Carbon Spheres (unbroken) 0.5 microns, gf m'g rg g 3:2 6. compositionaccording to claim 5 in which the Binder of Example 1 11.5 carbide iszirconium carbide.

7. A composition according to claim 5 in which the carbide is hafniumcarbide.

Forty-two pounds of the propellant was tired in a full 8. A compositionaccording to claim 4 in which the scale motor. The motor developed 4,000to 8,000 additive comprises a mixture of high melting carbide pounds ofthrust and was found to be absent frequenand particulate carbon.

cies above 5,000 Hz. 9. A composition according to claim 8 in which thePropellant compositions absent the additives of the particulate carbonis selected from hollow, thin-walled invention do not burn stably unlessthe ammonium percarbon spheres and carbon flakes.

chlorate level is below 80% by weight. This lowers both 10. Acomposition according to claim 9 in which said the impulse and densityof the propellant. The propeladditive further includes carbon powder.lant composition of the invention containing the stabi- 1 1. Acomposition according to claim 9 in which the lizing smokeless additivespermits formulation with particulate carbon is in the form of hollowcarbon over 85% ammonium perchlorate to form a high denspheres having adiameter between 100 and 200 misity solid propellant which burns stablywith high specrons and a wall thickness from 2 m 8 microns. cific.impulse and without visible smoke. 12. A method for producing thrust inthe absence. of

it is to be realized that only specific embodiments of visible smokecomprising the steps of:

the invention have been described and that numerous burning a solidpropellant composition defined acsubstitutions, alterations andmodifications are all percording to claim 1 at a burning rate aboveabout missible without departing from the scope of the inven- 0.4 in/secwithout combustion instability at pressures of at least 1000 psia so asto produce nontion as defined in the following claims.

smoking combustion gases; and

What is claimed is:

1. A stable burning, solid propellant composition abexhausting saidgases through an orifice to produce sent visible smoke on burningcomprising a cured, intithrust. mate mixture of: 8 ,13. A compositionaccording to claim 2 in which the a major amount of solid inorganicoxidizing salt; butadiene polymer is selected from a carbo'xy-tera minoramount of a combustible synthetic, organic minated polybutadiene curedwith an amine or an epelastomeric hydrocarbon resin formed by the chainoxide, a butadiene-acrylonitrile-acrylic terpolymer extension andcross-linking of functionally-tercured with an epoxideandhydroxy-terminated polybuminated, liquid butadiene, polymers; andtadiene cured with a diisocyanate. 0.2 to 5% by weight of thecomposition of a stability 14. A-composition according to claim 11 inwhich the enhancing additive consisting essentially of the hollow carbonspheres are unbroken. combination of: a

i 0 i i i least about 2000C selected from the group con--

1. A STABLE BURNING, SOLID PROPELLANT COMPOSITION ABSENT VISIBLE SMOKEON BURNING COMPRISING A CURED, INTIMATE MIXTURE OF: A MAJOR AMOUNT OFSOLID INORGANIC OXIDIZING SALT; A MINOR AMOUNT OF A COMBUSTIBLESYNTHETIC, ORGANIC ELASTOMERIC HYDROCARBON RESIN FORMED BY THE CHAINEXTENSION AND CROSS-LINKING OF FUNCTIONALLY-TERMINATED, LIQUID BUTADIENEPOLYMERS; AND 0.2 TO 5% BY WEIGHT OF THE COMPOSITION OF A STABILITYENHANCING ADDITIVE CONSISTING ESSENTIALLY OF THE COMBINATION OF: I. ANOXIDE OR CARBIDE HAVING A MELTING POINT OF AT LEAST ABOUT 2000*CSELECTED FROM THE GROUP CONSISTING OF THORIUM, TUNGSTE, SILICON,MOLYBDENUM, ALUMINUM, HAFNIUM, ZIRCONIUM AND VANADIUM OXIDES ANDCARBIDES; AND
 2. PARTICULATE CARBON.
 2. particulate carbon.
 2. Acomposition according to claim 1 in which the binder is an elastomerichydrocarbon polymer present in an amount of no more than 15% by weightand the additive is present in an amount from 0.1 to 4% by weight.
 3. Acomposition according to claim 2 in which the binder is a chain extendedand cured liquid polybutadiene polymer having an equivalent weightbetween 1,000 and 5,000 and a functionality between 1.7 and 3.0.
 4. Acomposition according to claim 3 in which the oxidizing salt is ammoniumperchlorate present in an amount of between 85% and 90% by weight.
 5. Acomposition according to claim 4 in which the additive comprises 0.2 to1% by weight high melting carbide having a particle size ranging between2 to 10 microns.
 6. A composition according to claim 5 in which thecarbide is zirconium carbide.
 7. A composition according to claim 5 inwhich the carbide is hafnium carbide.
 8. A composition according toclaim 4 in which the additive comprises a mixture of high meltingcarbide and particulate carbon.
 9. A composition according to claim 8 inwhich the particulate carbon is selected from hollow, thin-walled carbonspheres and carbon flakes.
 10. A composition according to claim 9 inwhich said additive further includes carbon powder.
 11. A compositionaccording to claim 9 in which the particulate carbon is in the form ofhollow carbon spheres having a diameter between 100 and 200 microns anda wall thickness from 2 to 8 microns.
 12. A METHOD FOR PRODUCING THRUSTIN THE ABSENCE OF VISIBLE SMOKE COMPRISING THE STEPS OF: BURNING A SOLIDPROPELLANT COMPOSITION DEFINED ACCORDING TO CLAIM 1 AT A BURNING RATEABOVE ABOUT 0.4 IN/SEC WITHOUT COMBUSTION INSTABILITY AT PRESSURES OF ATLEAST 1000 PSIA SO AS TO PRODUCE NON-SMOKONG COMBUSTION GASES; ANDESHAUSTING SAID GASES THROUGH AN ORIFICE TO PRODUCE THRUST.
 13. Acomposition according to claim 2 in which the butadiene polymer isselected from a carboxy-terminated polybutadiene cured with an amine oran epoxide, a butadiene-acrylonitrile-acrylic terpolymer cured with anepoxide and hydroxy-terminated polybutadiene cured with a diisocyanate.14. A composition according to claim 11 in which the hollow carbonspheres are unbroken.